1. Field of the Invention
The present invention generally relates to an assembly of a window blind and, more particularly, to a cord-winding assembly of the window blind that allows the user to smoothly raise or lower a window cover of the window blind using less strength.
2. Description of the Related Art
Referring to FIG. 1, U.S. Pat. No. 6,283,192 discloses a flat spring drive system and a window cover having two spools 91a and 91b. Both spools 91a and 91b are in a conical form and have a large end with a relatively larger diameter, as well as a small end with a relatively smaller diameter. The two spools 91a and 91b are positioned upside down with respect to each other, such that the large end of spool 91a is aligned with the small end of spool 91b, and the small end of spool 91a is aligned with the large end of spool 91b. A spiral groove is formed on an outer surface of each spool 91a/91b and extends from the large end to the small end thereof. A cord 92 is wound along the spiral grooves of the spools 91a and 91b. 
During use, the flat spring drive system is installed in the housing of an upper head rail 93. Upper head rail 93 includes a spring unit 95 connected to spool 91a via a gear unit 94, as well as a pulley 97 connected to spool 91b via another gear unit 96. In this structure, when the user raises a bottom rail of the flat spring drive system (not shown), spring unit 95 will drive gear unit 94 to rotate under its elastic force, thereby controlling the winding of cord 92 between spools 91a and 91b. At the same time, gear unit 96 is also driven so that pulley 97 is able to collect a lift cord “C” that extends through a plurality of slats of the window cover. In this regard, gear unit 94 controls spring unit 95 so that spring unit 95 is able to dynamically release sufficient elastic force to support the weight of the plurality of slats.
In the above structure, both spools 91a and 91b have a spiral groove 911 on its outer circumferential surface, so that cord 92 can be continuously wound around spools 91a and 91b along spiral grooves 911. However, each round of spiral groove 911 can receive only one turn of cord 92. Thus, when the length of the window cover is increased, the required length of lift cord “C” is also increased. In this case, the height of each spool 91a/91b should be increased so that a larger number of turns of lift cord “C” can be wound around spools 91a and 91b. However, since the interior space of the housing of upper head rail 93 is limited, the height of each spool 91a/91b cannot be increased as desired. In light of this problem, it is necessary to use gear units 94 and 96 in the flat spring drive system to provide a torque-changing effect. Based on the torque-changing effect, pulley 97 can be driven to dispense or collect the long cord (lift cord “C”) using the short cord (cord 92). Disadvantageously, use of gear units 94 and 96 will lead to an inconvenient assembly and an increment in cost of the system. In light of this, it is necessary to improve the conventional flat spring drive system.